A sample of low-redshift BL Lacertae objects. II.  EVN and MERLIN data and
multi-wavelength analysis




M. Giroletti (1 and 2), G. Giovannini (1 and 2), G. B. Taylor (3 and 4), R. Falomo (5)
((1) INAF Institute of Radioastronomy, (2) Univ. of Bologna, (3) Univ. of New Mexico, (4) NRAO Socorro, (5) INAF Observatory of Padua)




Accepted by ApJ. 17 pages.





We present new radio observations of 9 members of a sample of 29 nearby (z <
0.2) BL Lac objects. The new data have been obtained with the European VLBI
Network and/or the MERLIN at 1.6 and 5 GHz and complement previous
observations. For one object, the TeV source Mrk 421, we also present deep
multi-epoch VLBA and Global VLBI data, which reveal a resolved diffuse jet,
with clear signatures of limb brightening. We use the new and old data to
estimate physical parameters of the jets of the sample from which the subset
with new radio data is drawn. We derive Doppler factors in the parsec scale
radio jet in the range ~2 < delta < ~9. Using HST data, we separate the
contribution of the host galaxy from that of the active core. From the measured
and de-beamed observables, we find a weak correlation between radio power and
black hole mass, and a tight correlation between radio and optical core
luminosities. We interpret this result in terms of a common synchrotron origin,
with little contribution from a radiatively efficient accretion disk. The BL
Lacs in our sample have de-beamed properties similar to low power radio
galaxies, including the fundamental plane of black hole activity.